As it comes from the ground, crude oil sometimes contains a substantial proportion of water which is in the form of discrete droplets dispersed in the oil; the oil is the continuous phase. Such crude is often referred to as "wet crude." The water may be present for any of several reasons. The rise of the oil-water contact level in the underground oil producing formation may give rise to "coning" of water at high well flow rates; or water may be continuously admixed with the oil if the water level covers part of the producing zone. Where the oil is in a stratified formation zone, "lenses" of water may be trapped within the oil zone and be produced with the oil until all the water has been depleted.
Whatever its source, it is desirable to separate such admixed water before the crude is refined. Moreover, in many or most cases the water contains dissolved salt which itself is a serious detriment. High salt content may prevent bulk transportation of wet crude; and in any event salt causes corrosion in flow lines, separators and storage tanks.
Separation of admixed water is complicated by the fact that the water is generally in emulsion form. Emulsification occurs as the water travels with the oil through the well conduit, flow lines and degassing facilities. The constant agitation of the mixture in the well bore during production, and above ground in flow lines and processing vessels, causes the water to emulsify; release of dissolved gas as the line pressure drops can also contribute to emulsification. The emulsified water is often in the form of small droplets which have high surface tension and can be very difficult to separate by normal (gravity) settling. The emulsion can be especially tight and difficult to break if the proportion of emulsified water is small.